Vacuum-thermoforming methods for producing plastic articles are well known to those skilled in the art. These methods use the atmospheric pressure of air to force a heated sheet into an evacuated space. Thus, for example, vacuum thermoforming is described in Brage Golding's "Polymers and Resins," (D. Van Nostrand Company, Inc., Princeton, N.J., 1959), pp. 610-615. Vacuum thermoforming is also described in Joseph F. Hanlon's "Handbook of Package Engineering," Second Edition (McGraw-Hill Book Company, N.Y., 1984), pages 8-71 to 8-84. The disclosure of said Golding and Hanlon books is hereby incorporated by reference into this specification.
In the vacuum thermoforming process, a thermoplastic material is heated until it is formable, contacted with a mold, forced into an evacuated space where it is formed, and then removed from the mold. If the formed thermoplastic material is removed from the mold too readily, then, depending upon the thermoplastic material used, it might undergo a substantial amount of shrinkage. If, however, it is allowed to stay in contact with the mold until it has had more time to cool in order to minimize shrinkage, the mold cannot be used during the cooling cycle, and productivity suffers. To the best of applicant's knowledge, the prior art does not provide a simple, inexpensive machine which minimizes shrinkage of the molded product, allows high productivity, and can easily be incorporated into preexistant conventional machine systems/lines.
U.S. Pat. No. 3,673,834 of Brunner et al. discloses a forming apparatus comprised of lower wheel 22 and upper wheel 23, one of which carries a plurality of female dies and the other of which carries a plurality of male dies. Metallic foil is passed between the lower wheel and the upper wheel. At a specified point in each cycle, which is different for each molding plate, the molding plate is pushed by mechanical means towards the pressure plate, thereby compressing the metallic foil. The molding plate is removed from the pressure plate, and the molded metallic object is removed from the mold. Because the process of the Brunner et al. patent does not appear to be involved with thermoplastic materials, shrinkage does not appear to be a substantial problem. If, however, it were possible to adapt said process for use with thermoplastic materials, shrinkage might be a substantial problem, because the molding plate must be removed from the pressure plate and the formed article must be ejected in order to continue the revolution of the upper and lower wheels. If the process is operated so that the molding plate is not removed from the pressure plate until the molded article has cooled, then productivity will be poor. If, on the other hand, the molding plate is removed from the pressure plate immediately upon the forming of the molded article, then productivity of a molded article with a substantial amount of shrinkage will be high.
U.S. Pat. No. 3,673,835 of Arfert discloses an apparatus similar to that disclosed in U.S. Pat. No. 3,673,834 for use in forming containers from sheet material. In the process of the U.S. Pat. No. 3,673,835, a fluid under pressure is used to move the molding plate towards and against the blank to be formed. As is the case with the 3,673,843 patent, even if the process of this patent could be adapted for use with thermoplastic sheet material, the process would either produce molded articles with substantial shrinkage and/or would have to be run at a very slow speed in order to minimize shrinkage.
U.S. Pat. No. 4,413,964 of Winstead discloses an apparatus for extruding and biaxialy orienting a thermoplastic material. The web of thermoplastic material so formed is passed in contact with a polygonal molding plate with multiple molds in it. The use of a pressure plate with the molding plate is not disclosed, and the patent does not disclose how the thermoplastic material is fed to, formed by, or dispensed by the machine. It appears that, because of said lack of a pressure plate rotated in synchrony with the molding plate, the molded thermoplastic objects will lack suitable definition and uniformity. It will be difficult, if not impossible, to make parts with complicated shapes and/or thick parts with the machine disclosed by this patent.
U.S. Pat. No. 4,284,396 also discloses a forming apparatus which lacks any kind of an ongoing pressure plate system. The apparatus of this patent comprises a round mold drum system and a device for holding the thermoplastic material on the drum while the material is being heated and formed on the drum's molding surface as it advances. As is the case with the process of the U.S. Pat. No. 4,413,984, the process of this patent is not suitable for producing thick parts or parts with complicated shapes or any considerable depth and slight draft.
It is an object of this invention to provide a vacuum thermoforming process which, for a given thermoplastic feed stock and a given degree of productivity, will have substantially lower shrinkage.
It is another object of this invention to provide a vacuum thermoforming process which, for a given thermoplastic feed stock and a specified amount of shrinkage to be tolerated in the process, will run at a substantially higher speed and have substantially greater productivity. This feature is especially desirable when working with heavy guage thermoplastic materials.
It is yet another object of this invention to provide a vacuum thermoforming process which, although it is operated at relatively high speed, produces a molded product which is substantially more uniform.
It is yet another object of this invention to provide a vacuum thermoforming process which is substantially more suitable for producing molded parts with added height.
It is yet another object of this invention to provide a process which produces molded parts with good definition.
It is yet another object of this invention to provide a forming system which can use many, if not all, of the forming techniques which might be required in any particular case to form the precise formed part required.